Funds for the purchase of a modern X-ray diffraction system (consisting of a rotating anode X-ray source, diffractometer, associated computer and graphics system) are requested. This X-ray diffraction system will be used to collect data, solve, refine and graphically depict structural results in support of several NIH-supported projects. The requested hardware will enhance structure determination capabilities by providing a more intense X-ray source to reduce data collection times, improve signal to noise ratios for marginal crystalline specimens and allow for more careful preliminary examination of crystals. Molecular structure determinations are planned for a variety of chemical species related to natural products and biochemical systems. Studies of metalloporphyrin complexes will relate basic structural and physicochemical data to the function of hemoproteins that are fundamental to a wide range of biological processes including oxygen utilization and transport, electron transport, drug metabolism and other enzymatic processes. The molecular structure and solid-state conformations of modified nucleosides will be studied in order to assess the effect of furanose ring structure on overall conformation. Structure determinations will be performed to support new synthetic methods in the synthesis of streptogramin and Beta-lactam antibiotics. Such studies have proved to be important in understanding biological activity, provide structural evidence of synthetic intermediates, and suggest alternate synthetic routes to yield desired products. Structures of siderophores and siderophore-antibiotic conjugates are expected to provide a better understanding of the active and inactive forms of these drug and drug-delivery systems. Molecular structure determination will also be performed in support of research programs concerned with the development of new synthetic methods of pharmacological significance.